Smart thermostats can save substantial heating and cooling energy. The amount you save depends on your home’s performance, you heating/cooling equipment, the quality of the smart “t-stat”, and your willingness to let your home be a bit cooler in winter at a few points in time (or all the time) than the temperature you prefer. And the saving will vary in summer for the same factors including your willingness for your home to be a bit warmer and/or more humid in summer.
Smart t-stats have a wide range of capability and represent the latest stage in the evolution of thermostats. They are not only programable and Wi-Fi-enabled, but much more.
The fundamental attribute is that they allow you to program them so that your heating/cooling system operates only exactly when you need it to operate. And no longer. And also they are better at achieving exactly the temperature you select instead of veering too high or low.
Part of this is as simple as being more intuitive to set than many other t-stats so that programming is easier. However, a smart t-stat goes further and will also learn your habits and trim the time your heating and cooling system operates.
Another capability is that a smart t-stat can communicate via Wi-Fi with you and sometimes with other appliances, devices, plugs, and lights in your home allowing you to remotely reduce their energy consumption, not just trimming space heating and cooling.
Many sense your smart phone or tablet’s presence and automatically adjust the heating or cooling. For example, if you leave home for work early, the smart t-stat will turn the heat down earlier than you programmed. Or come home early, it senses when your smart phone is a few miles (or whatever distance you set) from home and starts heating up your home anticipating your arrival. [Note: this capability is in addition to sensing you are approaching the smart thermostat. Many models default to turn off their display to save electricity, but turn their display on as you approach anticipating you may want to see the settings and other information.
The best smart t-stats learn your patterns to trim heating and cooling system operation beyond your programming.